1. Field of the Invention
This invention relates generally to thin film magnetic memory disks and related planar devices; and to a procedure for their manufacture which permits the use of less expensive materials than prior art, simultaneously greatly reduces the use of process water and chemicals, and also greatly reduces the need for the treatment and disposal of waste chemicals.
2. Description of Related Art
Hard magnetic disks are used to store digital information utilized for data processing. An advantage of such a disk is that it can provide high-speed random access. That is, one can either write or retrieve information from any selected area on the magnetic memory surface without having to serially traverse the full memory space of the disk. Generally, a hard magnetic disk is mounted within a disk drive which is akin to a record turntable in that it includes means for rotation of the disk and means for translating a head across the surface of the disk to provide access to a selected annular track. Typically, a plurality of disks are mounted on a single spindle in spaced relationship to one another and heads are provided to interact through their magnetic fields with oppositely close planar surfaces of each of such disks. With high density data storage made possible by the use of newer design heads and close flying heights, a single head and single disk surface may suffice for some applications. For planar magnetic storage devices such as cards, direct sliding contact by the head may be preferred.
The hard disks now available for memory applications are typically coated with a magnetic storage layer. Each of the disk surfaces which receives and stores information has a thin layer of magnetic material carried by a substrate. The heads which interact with each of the surfaces are so-called “flying” heads i.e., they do not touch the surface of the disk during its rotation—rather, they ride on an air film which acts as a bearing between the disk and the head. The head typically includes a magnetic coil to permit interacting with the magnetic film through the intervening air film space. The air film prevents wear of the head and the thin magnetic layer on the disk surface which would otherwise be caused by a contact between the head and the surface film. Other devices based on magnetic memory storage may not require an air bearing but may instead utilize a head which contacts the magnetic layer in a linear sliding manner.
Details of the construction of thin film magnetic media are given in U.S. Pat. No. 5,405,646 and are included herein for reference. Media are built up in layers, each of which performs a specific task. As shown in FIG. 1, the basis metal of the disk is generally an aluminum alloy, typically 0.030 inches thick for 2 inch diameter disks or 0.050 inches for 3.5 inch diameter disks. Disk alloys generally contain about 4 to 5 weight percent magnesium to add strength to the disk. Because these alloys are soft, a hard surface is built up by adding a coating of nickel-phosphorus alloy (88Ni-12P, weight percent basis as a typical example) by the immersion process known as electroless nickel plating.
(Note: The designation of a commonly encountered electroless nickel composition as 88Ni-12P (on a weight percentage basis) is used herein to avoid any ambiguity, but is not intended to be limiting, since the frequently used “NiP” suggests equiatomic nickel and phosphorus i.e. a compound rather than an alloy.)
The 88Ni-12P layer is typically 300 microinches thick after polishing to obtain a smooth surface. The hard 88Ni-12P layer is a firm base which provides support to much thinner subsequently added magnetic layers. The 88Ni-12P resists mechanical damage which might be caused by inadvertent contact impacts between the head and disk surface, also known as “head slap”.
As head flying or glide heights have been lowered to one microinch or less to accommodate increased data density, the aluminum alloy substrate itself has recently been given initial polishing to minimize surface roughness. The premise for pre-polishing of the substrate is that a smooth starting surface for the substrate will yield a smooth 88Ni-12P deposit. However, the mechanical pre-polishing action tends to produce regions at the substrate surface which, although mechanically smooth, may respond unevenly in the wet chemical steps which condition the substrate for electroless nickel plating according to the prior art.
The uneven response of the highly polished substrate has been termed “carpeting” or “wall” effect and is believed to be the result of cold-working of the soft aluminum alloy. Even without pre-polishing, aluminum alloy substrates become roughened as a result of etching during immersion in prior art pre-treatment baths. The “carpeting” effect adds further roughening. The exact mechanism which causes carpeting is not well understood, particularly in the way that mechanical cold work influences chemical behavior. There is presently no theoretical or experimental evaluation known to applicant of an inherent difference in electrochemical activity between stressed and unstressed aluminum alloy. Further, the growth (and dissolution) of the ever-present thin layer of aluminum oxide which forms rapidly and naturally in air may be influenced by the cold-worked material. A full study of “carpeting” may require examination of the cold-worked surface for imbedded particles of polishing compound or for the presence of amorphous non-crystalline regions of the aluminum alloy. However, the present invention diminishes the problem by covering over surface variations and by providing for the nucleation of electroless 88Ni-12P growth without resorting to chemical pre-treatments which accentuate “carpeting”.
The present invention overcomes the problem of “carpeting” of electroless 88Ni-12P and also serves to reduce the cost of memory disk manufacture by incorporating the results of experimentation and recent technical advances in materials science together with the general method described earlier by Nanis in U.S. Pat. No. 5,405,646.
The major steps of coating a disk with the several layers necessary for a thin film memory disk in accordance with the prior art are shown in FIG. 2. The aluminum alloy substrate (disk) is degreased, washed in an alkaline soap solution and then rinsed in water. It is then etched in a dilute mineral acid bath and is then rinsed. The surface is then prepared for electroless nickel plating of the 88Ni-12P layer by a double zincating process. The above-mentioned sequence is included for example only and has many variants such as single or triple zincating.
Glass, polished to a smooth finish, has been favored as an alternate substrate to replace aluminum alloys. As experience with glass substrates has accumulated, new problems have been recognized, some of which have been overcome by the addition of a layer of electroless nickel (88Ni-12P) on the glass. Starcke et al., U.S. Pat. No. 5,871,810, recommend a multi-step chemical procedure to activate glass, ceramic and glass-ceramic substrates for electroless nickel plating, involving dipping a substrate in a solvent containing a metallo-organic source of palladium followed by baking at 200 C to 600 C to remove solvent and fix an adhesion layer on the glass which is catalytic for the nucleation of electroless nickel deposition. Starcke et al. indicate that a layer of electroless nickel is desirable to completely encapsulate and seal a glass substrate in order to overcome a corrosive effect of naturally occurring alkaline metal ions in glass, termed “salt bloom”. Starcke et al. also indicate that an encapsulating layer of 88Ni-12P beneficially provides an easily polished top layer, thereby overcoming polishing difficulties inherent with hard ceramic and glass-ceramic substrates.
Ross has addressed yet another difficulty encountered in the use of glass substrates for memory disks, namely the need to provide a textured region on the disk surface to aid the intended operation of flying heads and to prevent “stiction” when a head lifts off from the surface (U.S. Pat. Nos. 5,741,560, 6,143,375). Ross indicates that glass is difficult to texture controllably by chemical etching and is also difficult to texture by localized laser action. In U.S. Pat. No. 5,741,560, Ross describes a solution to the problem of laser texturing by providing a metallic initiation layer on an unpolished glass substrate which, in turn, initiates the growth of a polishable and texturable metallic layer by the process of electroless deposition. The general approach taken by Ross to solve the problem of laser texturing of glass substrates was earlier revealed by Nanis in U.S. Pat. No. 5,405,646, including materials for the initiation layer, method of sputtering to apply said initiation layers and subsequent electroless deposition of 88Ni-12P.
It will be appreciated by those skilled in the art that the teaching of U.S. Pat. No. 5,405,646 (issued 11 Apr. 1995) provides for a general application unlimited as to the selection of substrate material. Electroless 88Ni-12P has been found to be a useful top layer for glass substrates and it continues to be a practical choice for aluminum alloy substrates of the 5086 class. As mentioned, other strong, non-magnetic, polishable metal substrates may be considered for memory disk applications when coated according to the present invention, thus offering the opportunity for substantial cost saving.